Utilization of personal computers in business and scientific applications has proliferated considerably over the past decade. There is increasing use of small, single-function systems, such as word processors and small business computers. To share and exchange data between system, and to share resources, systems have been constructed as a centralized network having a single centralized controller operating multiple terminals in a time shared manner. This has evolved into a so-called distributed network which divided tasks among computing facilities, each termed a "node"; the intelligence residing at each node is termed a "host".
Distributed networks can be organized in a number of ways, either globally covering great distances or they can be local area networks (LAN) covering relatively short distances. A LAN ordinarily will consist of computers distributed within an office, a building or a campus. More information on local area networks is available in numerous texts, such as Keiser, Local Area Networks, second edition, McGraw Hill, Inc., 1989.
Three basic methods to connect local area networks to other networks are bridgers, routers and gateways. Bridges are data-link layer devices that connect similar networks together, e.g., Ethernet to Ethernet. Routers operate at Layer 3 of the OSI model, performing alternate routing by maintaining a routing table in each host. Gateways operate at the higher OSI Layers, carrying out protocol conversion and other connectivity functions. The term "gateway" will be used hereinafter as a term generic to bridgers, routers and gateways.
Personal computers or local area networks are interconnected by a transmission medium, such as coaxial cable, twisted wire pairs and optical fibers, depending principally on the rate of data transfer required. The present invention utilizes Integrated Services Digital Network (ISDN) to enable local area networks to be interconnected with each other to form a wider area network or to enable a local area network to be accessed by a remote personal computer and operated as if the computer were resident on the network.
ISDN is a relatively newly developed and emerging field of telecommunications which integrates computer and communications technologies to provide, worldwide, a common, all-digital network. This is based, in part, on standardizing the structure of digital protocols developed by the International Telegraph and Telephone Consultative Committee (CCITT). Despite the implementation of multiple networks within national boundaries, from a user's point of view there is a single, uniformly accessible, worldwide network capable of handling a broad range of telephone, data and other conventional and enhanced services.
A complete description of the architecture of ISDN is beyond the scope of this specification. For details, and for an extensive bibliography of references on ISDN, see Stallings, ISDN, An Introduction, MacMillan Publishing Company, 1989.
ISDN is structured by architecture closely following the OSI Seven Layer Reference Model. Within the framework of ISDN, the network provides services and the user accesses the services through the user-network interface. A "channel" represents a specified portion of the information carrying capacity of an interface. Channels are classified by two types, Basic Rate ISDN (BRI) and Primary Rate ISDN (PRI). BRI delivers two B-channels, each having a capacity of 64Kbps, capable of transmitting voice and data simultaneously. A 16Kbps D-channel transmits call control messages and user packet data. PRI provides twenty three B-channels of 64Kbps capacity each for carrying voice, circuit switched data or packet data. The D-channel is a 64Kbps signaling channel. The B and D channels are logically multiplexed together at Layer 1 of the OSI Reference Model.
FIG. 1 depicts the conventional ISDN interfaces. At the customer premises, an "intelligent" device, such as a digital PBX terminal controller or LAN, can be connected to an ISDN terminal TE, such as a voice or data terminal, which is connected to a Network Termination (NT1). Non-ISDN terminals TE may be connected to a Network Termination (NT2) and a Terminal Adapter TA. The NT2 in turn is connected over an "S/T-Interface", which is a four-wire bus, to a termination NT1 that performs functions such as signal conversion and maintenance of the electrical characteristics of the loop.
At the local loop, a two-wire bus, termed the "U-Interface", or "Loop", interconnects NT1 and a Loop Termination (LT) at the central office. Finally, the "U-Interface" is a bus between the local loop at the carrier end and exchange switching equipment. Details of this architecture are provided in ISDN: An Overview, Data Pro Research, Concepts & Technologies, MT 20-365; pp 101-110, published by McGraw Hill, Incorporated (December 1988).
An ISDN gateway must satisfy several functions. It must be capable of transferring files on the ISDN at a very high rate of data transfer. The gateway furthermore must be "transparent" to the user, that is, the user of a computer should not be able to distinguish between data transfer among local resources and remote data transfer over the ISDN. Furthermore, standard communications software available for personal computers should be application to communications on the ISDN. Bandwidth utilization of the ISDN line must be efficient and independent of the amount of traffic encountered. Preferrably, to enable local area networks anywhere to access an ISDN line, gateway hardware and programming will be resident at customer premises equipment, rather than at the central office.